A variety of battery-powered portable electronic apparatuses, including electronic wristwatches and mobile phones, have been introduced into the market. These electronic apparatuses are equipped with high power consuming loads, such as a motor, a buzzer, an illumination device, etc., in order to implement the required functions. Batteries are used as power supplies to drive these high power consuming loads. Most of the batteries used as power supplies in such portable electronic apparatuses are secondary batteries. In recent years, high-performance batteries that are small in size, but large in capacity, such as lithium-ion batteries, have been developed.
However, the performance and functionality of such portable electronic apparatuses has been increasing rapidly, and the number of loads mounted therein has also been increasing. On the other hand, portability and aesthetics are important considerations in the design of portable electronic apparatuses. For example, electronic wristwatches are becoming increasing thin and light-weight. As a result, batteries that can be mounted in them have to be made smaller and thinner, resulting in a situation where the battery capacity decreases and the battery output impedance degrades when the number of loads to be driven is increasing. In this way, the power supply design for portable electronic apparatuses is becoming increasingly constrained, and there is therefore a need for a power supply system that controls the battery as a power supply and the driving of a load in a more efficient manner.
In view of the above situation, it is known to provide a battery-powered electronic apparatus equipped with a voltage judging means that judges the recovery state of a battery after driving a load (refer, for example, to Patent Document 1).
The prior art voltage judging method disclosed in Patent Document 1 will be described below. FIG. 14 is a block diagram showing the configuration of a paging receiver equipped with a voltage judging means as disclosed in Patent Document 1, and FIG. 15 is a characteristic diagram of a battery mounted in the paging receiver.
The paging receiver 50 is an electronic apparatus equipped with the prior art voltage judging means, and has a function to receive a paging signal addressed to it and to display the received information. A reception processing unit 51 detects and demodulates a radio signal received via an antenna ANT. An ID-ROM 52 is a nonvolatile memory for storing address data, etc. of the paging receiver 50. A signal processing unit 53 drives the reception processing unit 51 intermittently.
A control unit 54 is constructed from a CPU, etc., and carries out processing, such as incoming call processing and voltage judgment processing, by controlling the various units in the paging receiver 50 in accordance with a control program stored in a ROM 55. A RAM 56 is used as a working area by the control unit 54, and stores various register and flag data. A voltage measuring circuit 57 measures the voltage of the battery 63, and generates a detection signal Sd when the voltage Vd of the battery 63 drops below a voltage V12 (to be described later) or a detection signal Su when it exceeds a voltage V11 (to be described later).
A display driver 58 receives display data from the control unit 54, and displays the data on a display unit 59 constructed from an LCD panel or the like. A key input unit 60 is constructed from various key switches (for example, a power switch and a reset key), and generates a switch signal that matches the operation of each key switch. A driver 61, in response to an annunciator driving signal supplied from the control unit 54, drives a speaker 71 that produces an alerting sound indicating the arrival of an incoming call, a vibrator 72 that produces vibration to indicate the arrival of an incoming call, and an LED 73 that illuminates when an incoming call arrives. A voltage raising circuit 62 is constructed from a DC/DC converter, and raises the voltage of the battery 63 for output during high load operation. The high load operation here refers, for example, to the operation in which the driver 61 drives any one of the annunciators, the speaker 71, the vibrator 72, or the LED 73, when an incoming call arrives.
Next, the voltage judging operation in the paging receiver 50 will be described with reference to FIGS. 14 and 15.
First, when the power switch on the key input unit 60 is turned on, the control unit 54 performs initialization and enters an intermittent receive mode to wait for the arrival of an incoming call. When an incoming radiowave is detected, thus detecting the arrival of an incoming call, the address data of the detected incoming radiowave is compared with the address data stored in the ID-ROM 52 to determine whether they match or not. If the two addresses match, processing is performed to capture the received data into a receive buffer. Upon recognizing, as a result of the signal capturing processing, that the paging is addressed to the paging receiver 50, the control unit 54 supplies an annunciator driving signal to the driver 61 which thus drives the speaker 71, the vibrator 72, or the LED 73 to annunciate the incoming call.
When the high load operation for driving the speaker 71, the vibrator 72, or the LED 73 is performed in this way, the voltage Vd of the battery 63 begins to drop at time t101 at which the high load operation is started, as shown in FIG. 15. The battery voltage Vd drops to the voltage V12 at time t102 at which the high load operation ends. At the high load operation end time t102, the internal timer of the control unit 54 is started. At voltage judging time t103 when a predetermined time has elapsed, the voltage measuring circuit 57 measures the battery voltage Vd.
Starting from the high load operation end time t102, the battery voltage Vd gradually recovers and rises. At the voltage judging time t103, if the battery voltage Vd is higher than the voltage V11 (as indicated by the voltage characteristic BT1), the operation continues by returning to the incoming call waiting state. On the other hand, at the voltage judging time t103, if the battery voltage Vd is not higher than the voltage V11 (as indicated by the voltage characteristic BT2), it is determined that the battery 63 is unable to drive the load. Then, a message saying, for example, “BATTERY IS LOW. REPLACE BATTERY.” is displayed on the display unit 59, urging the user to replace or recharge the battery, while stopping the operation of the reception processing unit 51.
In this way, the prior art electronic apparatus equipped with the voltage judging means disclosed in Patent Document 1 checks the voltage recovery state when a predetermined time has elapsed after stopping the high load operation. Accordingly, the prior art electronic apparatus can avoid to a certain extent the problem of immediately stopping the operation or urging the user to replace the battery by determining that the battery is low even when the battery voltage just drops temporarily.
The electronic apparatus equipped with the prior art voltage judging means disclosed in Patent Document 1 judges the voltage after a predetermined time (voltage recovery period) has elapsed after stopping the high load operation. Accordingly, when the battery is in a nearly fully charged condition, for example, since the battery recovers quickly after stopping the high load operation, time loss occurs in judging the recovery state of the battery. As a result, when driving a load repeatedly, or when driving a plurality of loads sequentially in succession, there arises the serious problem that the load or loads cannot be driven quickly.
Furthermore, since the voltage recovery characteristic of the battery after stopping the high load operation greatly varies depending on ambient temperature and other load driving conditions, it is difficult to accurately judge the recovery state of the battery with the prior art judging method that measures the voltage only once after the predetermined time (voltage recovery period) has elapsed. Such a judging method requires that the detection margin be increased in order to compensate for the poor accuracy in the detection of the battery recovery state. However, if the detection margin is increased, the voltage recovery period has to be set longer, which makes it further difficult to achieve high speed load driving, and an electronic apparatus having excellent response characteristics cannot be achieved. If such a prior art power supply system is applied to a portable electronic apparatus that requires a compact and thin design, the operation will become unstable because of insufficient capacity of the power supply, and it is extremity difficult to achieve a highly reliable product.
It is also known to provide a battery-driven power tool equipped with a battery-driven power source (motor) and having a remaining capacity detection circuit that detects the remaining capacity of the battery (refer to Patent Document 2). This remaining capacity detection circuit judges the remaining capacity of the battery by detecting whether the time rate of change of battery voltage when a predetermined time has elapsed after power is turned off to the power source exceeds a predetermined value. Accordingly, in the battery-driven power tool disclosed in Patent Document 2, the remaining capacity of the battery can be detected with relatively good accuracy by minimizing the effects due to the condition (such as the load condition) of the power source.
The electronic apparatus disclosed in Patent Document 2 is designed to judge the remaining capacity of the battery by the time rate of change of battery voltage when a predetermined time has elapsed after power is turned off to the power source. However, in the electronic apparatus disclosed in Patent Document 2, when, for example, the battery is in a nearly fully charged condition, and the battery recovers quickly after turning off power to the power source, time loss occurs in judging the recovery state of the battery, as in the case of the electronic apparatus disclosed in Patent Document 1. Accordingly, the electronic apparatus disclosed in Patent Document 2 involves the same problem as the electronic apparatus disclosed in Patent Document 1, that is, the power source such as the motor cannot be quickly operated repeatedly.    Patent Document 1: Japanese Unexamined Patent Publication No. 2000-156722 (page 4, FIG. 1)    Patent Document 2: Japanese Unexamined Patent Publication No. 2003-25252 (page 6, FIG. 2)